Power System Protection

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Power System Protection

Dr. Lionel R. Orama Exclusa, PE Week 1

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Jan 18/2006 Copyright L.R. Orama 2006 2

Philosophy of Protective Relaying

Readings-Mason Chapter 1 • Relay function

• What exactly is a relay? • CT’s and PT’s

• Abnormal conditions-faults

• Desirable relay characteristics • Zones of protection

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Relay function

• Causes fast disconnection of equipment from power system during abnormal

conditions

– Minimizes damage

– Minimizes effects on the system operation – Maximizes power transfer

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Copyright L.R. Orama 2006 4

Relay function

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What exactly is a relay?

• Measuring device-receives a signal from the Power System (PS) & determines if the condition is abnormal

• Control device-for abnormal conditions, it signals Circuit Breakers (CB) to

disconnect equipment

– It kind of “relays” a signal from the PS to the CB – Relay & CB is a system protection team

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Current Transformers (CT’s)

• Device that produces current proportional to current in the power system conductor

• Ideal CT has current of:

• Rated I_CT is aprox. 5A, typically (called CT secondary current) CT SYS CT N I I =

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Current Transformers (CT’s)

400 20005 200 1000:5 120 600:5 80 400:5 40 200:5 20 100:5 10 50:5 Turns Current Common ratios

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Potential Transformers (PT’s)

• Device that produces voltage proportional to voltage in the power system bus

• Ideal PT has voltage of:

• Rated V_PT is aprox. 120V, typically (called PT secondary voltage) 1 2 N N V V_PT = _SYS

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Simple Contacts Diagram

• Short hand notation for the trip circuit

• Control Relaying & Instrumentation

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Relay Contacts

• “a” Contact

– Open when relay that controls it is not picked up, or when CB that controls it is open

– Closed when relay that controls it is picked up, or when CB that controls it is closed

• “b” Contact

– Closed when relay that controls it is not picked up, or when CB that

controls it is open

– Open when relay that controls it is picked up, or when CB that controls it closed

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Contacts

• A relay is said to be picked up when its:

– Its “a” contacts are closed or

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Faults (Short Circuits)

• Most common type of abnormal condition • Often Abnormally high currents flow I_F>>I_L • Flow of current in a path not intended for it • To interrupt I_F, circuit must be opened

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Faults (Short Circuits)

• Cause high current

– mechanical stress

• Cause high temperature arcs

– destroy equipment, cause fires

• Alter system voltage

• Cause system unbalance • Block the flow of power

• Cause generator overspeed

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Fault Current Interruption

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Desirable Relay Characteristics

• Speed (time to make decision 1/60sec)

– Minimizes damage from current

– Maximizes power transfer and stability

• Security

– Relay should not cause CB to open during normal conditions

• Dependability (reliability)

– Relay should cause CB to open during abnormal operation

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Desirable Relay Characteristics

• Sensitivity

– Ability to detect all faults for the expected limiting system and fault conditions

• Selectivity

– Ability to discriminate between faults internal and external to its intended zone of protection

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Primary & Backup Protection

• Relays, CB’s, CT’s & PT’s located at substations • Substation is a point of – Interconnection – Switching – Voltage transformation

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Primary & Backup Protection

• Every equipment should be protected by at least two independent protection

systems

• Equipment

– Lines, busses, transformers, generators

• Protection system

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Primary Protection

• Primary

– To do it, zones are drawn on the system’s one-line diagram

– Zones are defined by CT location and include equipment & CB’s that isolate equipment

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Primary Protection

• Primary protective zones

– CB’s located in connection to each PS equipment

– Zones configured so the fault within the zone causes all CB’s in the zone to open – CB’s open simultaneously &

instantaneously (no intentional delay) – If primary zone is

successful, minimum effect on the system (power

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Primary Protection

• Primary protective zones – Zones defined by the CT’s located in the bushings of the CB’s – Zones overlap to avoid uncovered areas and CB

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Primary & Backup Protection

Causes of Relay Failure • CT’s and circuits

• PT’s and circuits • Loss of DC supply

– Short circuit – Open circuit

– Failure of auxiliary switches

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Primary & Backup Protection

Causes of CB failure • Loss of DC supply • Open trip coil

• Short circuit trip coil • Mechanical failure

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Backup Protection

• Provided in case of:

– Primary relay failure – Circuit breaker failure

• Ideally primary & backup must be independent

– Relay, CB’s, CT’s, PT’s

– However, duplicity of CB’s is not practical & causes other problems

• Backup is slower than primary

• Opens more CB’s than necessary to clear a fault • Provides primary protection when primary

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Backup Protection

Types of backup protection • Remote backup

– Clears fault one substation away from where the failure has occured

• Local backup

– Clears fault in the same substation where the fault has occured

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Remote Backup

• CB’s P and O opens • CB M stay closed

• Then B, C and I has to open • Load is lost in this case

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Remote Backup

• Trips more system than necessary to clear the fault

• Long backup times from 50-60 cycles (0.8-1.0 sec)

• Difficult to set remote relays • Initially, more economical

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Local Backup

• Primary relay op. & energizes timer • If CB relays

remain picked up & timer times out

– Opens all CB’s on the bus (trips all CB’s)

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Local Backup

• J opens • H closed

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Local Backup

• P and O open • M closed • G and I opens • L trips locally • I tripped by communicated signal (Transfer Trip) • A special kind of backup keeps Z closed

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Local Backup

When a CB does not work

• Trips least amount of system in clearing a fault

• Fastest backup times are 9-20 cycles (0.15-0.33 sec)

• Simple relay settings

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Remote vs. Local Backup

• Remote

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Remote vs. Local Backup

• Remote